Many manufacturing applications rely on a heating, ventilating, and air conditioning ("HVAC") system to control environmental conditions such as temperature and humidity, or to provide ventilation or exhaust of hazardous fumes. In such systems, the air flow is typically controlled using dampers located at various points within ducts in the HVAC system. These dampers may be, for example, gate valves, butterfly valves, or blast valves, and may be fixed, adjustable or motorized. By increasing or decreasing the amount of airflow resistance or dampening, the airflow can be decreased or increased, respectively, in a particular region.
There are many factors, however, that contribute to the airflow rate in addition to the dampening, including the HVAC system pressure and the speed at which exhaust fans are operating. Furthermore, when the amount of dampening is changed in a particular damper, the pressure throughout the entire HVAC system will change, thereby affecting the flow rate through the other dampers in the system. While such variation is acceptable in some manufacturing applications, in others, such as minienvironments used in silicon wafer manufacturing or semiconductor integrated-circuit chip manufacturing, the amount of airflow and exhaust must be precisely controlled.
Minienvironments are used to house many of the process tool operations that are used in the silicon wafer and semiconductor integrated-circuit chip manufacturing industries. The typical minienvironment has two fundamental purposes. First, it provides the ability to maintain an extremely clean manufacturing space. Second, it provides containment and control of chemical vapors that are generated in many wafer-manufacturing and chip-manufacturing operations. It is well known that the delivery and control of the airflow through the minienvironment impacts both cleanliness and chemical containment. It is extremely important that the volumetric airflow though a minienvironment be controlled and maintained at a precise and continuous rate because 1) fluctuations in the airflow or exhaust can adversely affect the product being manufactured, and 2) the manufacturing process often involves the use of chemicals that produce hazardous fumes that must be ventilated to ensure the safety of operators and avoid damage to related machinery and systems. In many such applications, the amount of airflow should optimally not vary by more than +/-5 percent from a set-point.
The typical approach to regulating airflow uses dampers such as gate valves, butterfly valves, or blast valves in HVAC systems at the respective locations where the airflow is supplied to and exhausted from the minienvironment. The typical minienvironment is configured as shown in the FIG. 1 schematic. Supply air is supplied into the minienvironment by a fan-filter unit. As a person familiar with minienvironments would know, the fan in the module drives air through a suitable filter and then into the process area within the minienvironment. The air is exhausted through an exhaust duct to a scrubber (air may also be exhausted through other openings in the minienvironment). The exhaust duct may also have an adjustable damper.
In a typical minienvironment, air is not only driven through the minienvironment by the fan-filter unit, but also drawn from the minienvironment by a downstream exhaust fan. Both of these components affect the overall airflow in the minienvironment and must be adjusted relative to each other to obtain a desired equilibrium.
A wet bench is another example of a manufacturing application requiring precise control of airflow. A wet bench is a minienvironment process tool that is used to chemically clean silicon wafers. There are two basic airflow-balancing scenarios for a wet bench. One involves a standard "cool" tank where chemicals in the tank are generally maintained at a temperature below 90 degrees Celsius. In this scenario, desired airflow is achieved by first adjusting the exhaust damper to approximately 180 cfm. This step is followed by setting the fan filter speed such that the pressure inside the minienvironment is less than 0.001 in. w.g. but greater than 0.0002 in. w.g. The exhaust flow rate is then checked against the desired flow rate, and the process repeated if necessary. The second scenario in a wet bench involves a "hot" tank containing chemicals (typically phosphoric acid) that are maintained at a temperature above 160 degrees Celsius. In the second scenario, desired airflow is achieved by first adjusting the exhaust damper to 400 cfm followed by setting the fan filter speed to create the same range of positive pressure inside the minienvironment (i.e., less than 0.001 in. w.g. but greater than 0.0002 in. w.g.). The exhaust flow rate is then checked against the desired flow rate, and the process repeated if necessary. It should be understood that these settings are typical.
One problem with maintaining accurate airflow in systems is in balancing the airflow through different regions. The airflow in such a system is balanced by first adjusting the exhaust flow via the damper, and then adjusting the fan speed in the fan-filter unit until a slightly positive pressure is created inside the minienvironment. The balancing process typically requires repeatedly readjusting the dampers and fan speed in sequence until the flow rate reaches an equilibrium that is within an acceptable range. Such a balancing method is time consuming, tedious, prone to inaccurate settings, and not responsive to external changes in the airflow supply system. Furthermore, after the balancing is completed, the entire HVAC system is still subject to changes in supply or exhaust pressure and to changes in the demand requirements of various components of the HVAC system, any of which could require having to rebalance the system.
Another problem that arises in large manufacturing applications that have multiple minienvironments and complex HVAC systems, is balancing the overall system and providing feedback and information regarding the current state of operations to the equipment and facility operators. Due to many factors, a balanced process tool can eventually become "unbalanced" and corrupt the manufacturing process inside the minienvironment. Furthermore, it is typically not known how changes in damper or fan speed settings of a particular subsystem or minienvironment will affect the HVAC system in general, or particular subsystems. Such effects can only be determined by making individual measurements at discrete locations within the HVAC system, and making changes as necessary, which changes may cause still further effects at other locations within the HVAC system.
Another problem with some manufacturing applications is the presence of chemicals or fumes that are acidic or alkaline that can react with, or otherwise adversely affect, dampers or controls in the HVAC system. Such chemicals or fumes can, for example, corrode dampers or controls that are in contact with the chemicals in the airflow, causing them to operate incorrectly. In addition, the escape from the system of noxious gases can cause a safety problem for workers using or maintaining the equipment. Accordingly, there is a need to have a valve system capable of being completely sealed to prevent release of internal gases.
Another problem with some manufacturing applications is retrofitting existing systems to provide automated measurement and control systems. Many such systems have existing ducts of varying sizes. Accordingly, there is a need for a valve system that is modular and compact that can be installed in existing systems.
Consequently, there is a need for automated monitoring controls that can quickly and automatically respond to minor airflow fluctuations and make adjustments before the volumetric airflow rate deviates outside acceptable operating parameters. An important part of an automated control system involves the use of hardware that can physically adjust airflow quickly and accurately to respond to changes in the HVAC system or to needs of a particular minienvironment. There is also a need for such a system to be able to provide feedback and status information both locally and remotely, such as at a central processing location, so that the entire HVAC system can be monitored and controlled from a remote location.
There is also a need for the "wetted" components of such a control system to be resistant to damage that could be caused by chemicals likely to be present in the exhaust of particular manufacturing applications and to be able to provide a seal to prevent escape of such chemicals from the valve and control system. The "wetted" components are those components, such as the dampers, the interior of the duct, and the pressure sensors that are in direct contact with the airflow and chemicals that may be present in the airflow.
Finally, there is a need to be able to retrofit existing plants with such an automated control system. The invention disclosed below is an improved exhaust valve design that provides these capabilities.